1. Technical Field
The present disclosure relates to the fabrication of integrated circuit transistors, and, in particular, low-leakage three-dimensional FinFET (field effect transistor) devices.
2. Description of the Related Art
In a digital circuit, a transistor is a switch which ideally: a) passes zero current when it is off; b) supplies large current flow when it is on; and c) switches instantly between the on and off states. Unfortunately, a transistor is not ideal as constructed in an integrated circuit and tends to leak current even when it is off. Current that leaks through, or out of, the device tends to drain the battery that supplies power to the device. For many years, integrated circuit transistor performance was improved by shrinking critical dimensions to increase switching speed. However, as dimensions of silicon-based transistors continue to shrink, maintaining control of various electrical characteristics, including off-state leakage, becomes increasingly more challenging, while performance benefits derived from shrinking the device dimensions have become less significant. It is therefore advantageous, in general, to reduce leakage current in the transistor by alternative means, including changes in materials and device geometry
Integrated circuits typically incorporate FETs in which current flows through a semiconducting channel between a source and a drain, in response to a voltage applied to a gate. A traditional planar (2-D) transistor structure is shown in FIG. 1A and described below in greater detail. To provide better control of the current flow, FinFET transistors, sometimes called 3D transistors, have been developed, such as the one shown in FIG. 1B. A FinFET is an electronic switching device in which the planar semiconducting channel of a traditional FET is replaced by a semiconducting fin that extends outward, normal to the substrate surface. In such a device, the gate, which controls current flow in the fin, wraps around three sides of the fin so as to influence the current flow from three surfaces instead of one. The improved control achieved with a FinFET design results in faster switching performance and reduced current leakage.
Intel described this type of transistor in an announcement on May 4, 2011, calling it by various names including a 3D transistor, a 3-D Tri-Gate transistor, or a FinFET. (See, for example, the article titled “How Intel's 3D tech redefines the transistor” located on the internet at http://news.cnet.com/8301-13924—3-20059431-64.html; see also U.S. Publication No. 2009/0090976 to Kavalieros et al., published on Apr. 9, 2009; U.S. Pat. No. 8,120,073 to Rakshit et al.; U.S. Pat. No. 7,973,389 to Rios et al.; U.S. Pat. No. 7,456,476 to Hareland et al.; and U.S. Pat. No. 7,427,794 to Chau et al.)
An array of semiconducting fins is shown in FIG. 2. Typically, an array of multiple transistors can be formed by conformally depositing a common gate over an array of fins. Furthermore, an array of multi-gate transistors can be formed by conformally depositing multiple common gates over the array of fins. Such a FinFET array having three gates between source and drain regions is known as a tri-gate transistor.
Prior to the development of FinFETs, strained silicon transistors were developed to increase control of the mobility of charge carriers in the semiconducting channel. Introducing compressive strain into the transistor materials tends to increase charge mobility, resulting in a faster switching response to changes in voltage applied to the gate. Strain can be introduced, for example, by replacing bulk silicon in the source and drain regions, or in the channel itself, with epitaxially grown silicon compounds. The term epitaxy refers to a controlled process of crystal growth in which a new, epitaxial, layer of a crystal is grown from the surface of a bulk crystal, while maintaining the same crystal structure of the underlying bulk crystal.
Despite improvements provided by three-dimensional structures and strained silicon materials, transistors continue to suffer certain types of performance degradation as device dimensions shrink into the range of 1-50 nanometers. These include, in particular, leakage of charge between the semiconducting channel and the substrate.